Net Core - Assembly.GetEntryAssembly can return null when loaded from an unmanaged application

- Add null check and use currently assembly location to resolve libcef.dll

The GetEntryAssembly method can return null when a managed assembly has been loaded from an unmanaged application.
https://docs.microsoft.com/en-us/dotnet/api/system.reflection.assembly.getentryassembly?view=net-5.0